1. Field of the Invention
The disclosures herein relate to a scheduling method and a radio base station. The disclosures herein relate to a radio communication system using orthogonal frequency division multiplexing in which radio frequency resources are divided into three or more communication channels.
2. Description of the Related Art
In mobile communication systems utilizing OFDM (orthogonal frequency division multiplexing), the simultaneous use of overlapping radio frequency resources by adjacent cells causes mutual interference. Frequency allocation thus needs to ensure that different frequency resources are allocated to adjacent cells.
It is practically impossible, however, to divide frequency resources into a large number of resource portions that are fixedly allocated to respective cells in such a manner thus all the cells use different frequency resources. It is thus common to allow nonadjacent cells to repeatedly use overlapping frequency resources, thereby making recycling use of frequency resources.
In such frequency allocation system, FFR (fractional frequency reuse) has been attracting attention as a means to achieve further improvement in frequency utilization. FFR allows a frequency reuse distance to vary in response to the distance between a mobile station and a radio base station. As the mobile station moves farther away from the base station (thus suffering increased interference), the frequency reuse distance is increased to prevent interference between adjacent cells. As the mobile station moves closer to the base station (thus suffering less interference), the frequency reuse distance is decreased to improve frequency utilization. This arrangement is aimed at improving frequency utilization for the system as a whole.
Non-patent Document 1 discloses using FFR together with transmission power control for the purpose of further improving frequency utilization.
In this system, when a mobile station is situated far away from a radio base station (thus suffering large interference), a frequency band (priority channel) that is different from those of the adjacent cells is used. When the mobile station is situated close to the radio base station (thus suffering small interference), transmission power is reduced so as not to interfere with the adjacent cells, and a frequency band (non-priority channel) that is used by an adjacent cell is allocated. In this manner, transmission power is reduced to suppress interference with adjacent cells, thereby making it possible to use a frequency band that would not be used in a conventional system. With this arrangement, frequency utilization can be improved.
Further, Patent Document 1 discloses changing a frequency reuse distance in response to the distance between a mobile station and a radio base station in the FFR in which a default frequency reuse distance is 3. The frequency band is divided into four communication channels. Among these, three communication channels are used in a region where a frequency reuse distance is set equal to 3 for mobile stations situated near the edge of the cell, and the one remaining communication channel is used in a region where a frequency reuse distance is set equal to 1 for mobile stations situated near the center of the cell. With such provision, frequency utilization is improved while avoiding quality degradation caused by interference.
Patent Document 2 discloses dividing a cell into a center area and a surrounding area in a concentric fashion and suppressing transmission power to such a degree that the center area does not interfere with the adjacent cells when the same frequency is used in the cell of interest and the adjacent cells.
With regard to the allocation of a non-priority channel, Non-patent Document 1 only describes setting the transmission power to a level that does not affect the adjacent cells. No disclosure is given with respect to a method of controlling interference between the priority channel and the non-priority channel. Because of this, it is not possible to determine an optimum modulation and coding scheme (MCS), resulting in a drop of throughput.
There is thus a need for a scheduling method and a radio base station in which a suitable modulation and coding scheme can be selected for each mobile station to improve throughput.
[Patent Document 1] Japanese Patent Application Publication No. 2004-159345
[Patent Document 2] Japanese Patent Application Publication No. 2007-235201
[Non-patent Document 1] Samsung, “Flexible Fractional Frequency Reuse Appro,” 3rd Generation Partnership Project TSG-RAN WG1, R1-051341, 8.2, November 2005